1. Field of the Invention
The present invention relates to semiconductor protection components, realized in monolithic form. Such protection components are intended to clip a voltage applied to their terminals when this voltage exceeds a predetermined threshold. During the clipping phase, the voltage across the component is maintained at the predetermined threshold when the component is of the avalanche diode type or drops to a practically zero value when the protection component is a four-layer component of the break over type, such as a gateless triac.
2. Discussion of the Related Art
In FIG. 1 a protection component 1 is conventionally parallel connected to a line between a supply source 2 and supply input terminals 3 and 4 of a circuit 5 to be protected.
As indicated above, such components can be realized in various ways. FIG. 2 shows a structure of an avalanche diode formed on an N-type semiconductor substrate 10. The front surface area of this substrate includes a P-type region 11 and the rear surface area an N-type overdoped region 12. The upper surface of the substrate is coated with an isolating layer 13, usually silicon oxide, which serves to delineate the area where region 11 is formed. The upper surface is coated with a metallization 15 contacting region 11.
Conventionally, the lower surface of the substrate is mounted on a base 16 serving as a heat sink, by means of a solder preform 17 and metallization 15 is directly welded to a connection "nail".
A protection component as illustrated in FIG. 2 is intended to absorb overcurrents or overvoltages between metallization 15 and base 16. The size of this component is devised so as to absorb an overload up to a predetermined energy limit. However, there always exists an over-current level liable to destroy the component. Usually, such a destruction results from the occurrence of a shorting region 18 between the upper and lower metallizations. Then, once overvoltage is over, the component remains shorted and, in the case of a circuit such as shown in FIG. 1, circuit 5 is no longer fed between its terminals 3 and 4 because the source 2 is shorted. This situation constitutes a double drawback. To set circuit 5 into operation again, it is necessary to disconnect component 1 and, if it is desired to protect the circuit, component 1 must be replaced by a new component. This operation is always critical and, in some applications, such as for circuits used in satellites, requires the provision of a double amount of components and switching circuits that adversely affect the protection function quality.
To solve this problem, the practice is to use over-sized protection components capable of absorbing high overvoltages which are very unlikely to occur.